A microstrip line is a transmission line having a single signal line on one surface of a dielectric substrate and a single ground plane on the opposite surface of the substrate, and is a representative unbalanced line, which has been most widely used to connect circuits and parts within a unit. Meanwhile, a Coplanar Stripline (hereinafter referred to as a ‘CPS line’) is a representative balanced line, which has transmission lines formed parallel to each other in the same plane, and is widely used for the feed line of an antenna, a double-balanced mixer, a double-balanced multiplier, etc., which require the input of a balanced signal.
Meanwhile, in order to connect the unbalanced line to the balanced line in various types of RF/microwave circuits, a balun is generally used. The limits of frequency characteristics of the entire system of RF/microwave circuits are typically determined by a balun. However, a balun, which has been developed and used in the prior art as a structure for making a transition from a microstrip line to a CPS line, has the following problems.
Firstly, the operating frequency bandwidth thereof is limited. In the conventional balun, a mode conversion method or a coupling method is used to make a transition from a microstrip line to a CPS line, but the conventional balun has a limitation in that the optimal impedance matching between the microstrip line and the CPS line and the smooth transition of electric field distributions therebetween are not realized, and thus the frequency bandwidth is narrowed.
Secondly, in order to lower the characteristic impedance of a CPS line, a substrate having a high dielectric constant must be used. That is, to reduce the difference between the impedances of a microstrip line and a CPS line, a substrate having a high dielectric constant must typically be used, and thus there is a limitation when a substrate required for the implementation of a balun is selected.
Thirdly, the transition of the electric field distributions between a microstrip line and a CPS line is not smoothly made. That is, in a microstrip line, electric fields are generally distributed to be perpendicular both to a signal line and to a ground plane. In contrast, in a CPS line, electric fields are formed parallel to and between two transmission lines. Therefore, in these electric field distributions, the smooth transition between the two lines is essential, but the structure of the conventional balun cannot satisfactorily realize the smooth transition of electric field distributions.
Fourthly, a conventional ultra-wideband balun, which is used in an ultra-wideband antenna or an ultra-wideband double-balanced mixer, adopts the structure of parallel-plate stripline baluns, which use the top surface and the bottom surface of a dielectric substrate. However, in order to implement an ultra-wideband double-balanced mixer or the like using such parallel-plate stripline baluns, respective ports of diodes must be connected to the top surface and bottom surface of a substrate, so that there is a problem in that it is difficult to mount various elements thereon, and thus the costs required for the implementation of a mixer are increased. Further, in the case of an antenna requiring a balanced signal, one end of the antenna is placed on the top surface of a substrate and the other end thereof is placed on the bottom surface thereof, and thus this structure is not visually attractive, and in addition, the implementation of the antenna is inconvenient.